1. Field of the Invention
The present invention relates to a magnetic random access memory (MRAM). More particularly, the present invention relates to an MRAM using magnetic domain drag and giant magnetoresistance (GMR) or tunnel magnetoresistance (TMR), and a method of operating the same.
2. Description of the Related Art
MRAMs are nonvolatile memory devices and new solid magnetic memories using a magnetoresistance effect based on spin dependent conduction particular to nano-magnetic substances. MRAMs use GMR or TMR, which occur because a spin, the degree of freedom of an electron, exerts great influence on electron transport.
GMR is the phenomenon where there is a resistance difference in a multi-layered structure of ferromagnetic substance/non-magnetic metal substance/ferromagnetic substance between a case where the spin arrangements formed between the two ferromagnetic substances are the same and a case where the spin arrangements formed between the two ferromagnetic substances are opposite. TMR is the phenomenon where current more easily passes through a multi-layered structure of ferromagnetic substance/isolation substance/ferromagnetic substance in a case where the spin arrangements formed between the two ferromagnetic substances are the same than in a case where the spin arrangements formed between the two ferromagnetic substances are opposite. In the case of MRAMs using GMR, since the difference between resistance values according to a magnetization direction is small, the difference between voltage values cannot be increased. In addition, the MRAMs using GMR are disadvantageous in increasing the size of a MOSFET that is combined with a GMR film in order to form a cell. Accordingly, more research has been performed to put MRAMs using a TMR film to practical use.
In recording data in conventional MRAMs, the sum of vectors in X- and Y-directions is used to select a particular cell in an array having a matrix structure of unit MRAM cells. In other words, according to a method known as an asteroid curve, data is recorded taking into account the magnetization direction of a magnetic film with respect to the sum of external magnetic fields. In conventional MRAMs, current is applied to electrode lines crossing each other above a data storage unit (GMR or TMR) to induce a magnetization direction due to a switching field formed across the electrode lines into a free layer of the data storage unit, and the magnetization direction is used as an information unit.
In order to realize memory devices that can be put to practical use, it is necessary to minimize consumption of operating power, which limits the selection of a material used for a data storage unit. In a conventional MRAM, in order to reduce a switching field, i.e., consumed power, a Permalloy (NiFe) is most widely used for a free layer of a magnetoresistance device. It is essential to increase magnetoresistance in order to increase the operating speed of an MRAM and efficiently design a chip architecture. For this, a magnetic film used for a free layer must have a ferromagnetic characteristic and a high polarization characteristic. In order to obtain these characteristics, a NiCoFe alloy, half metallic alloy, magnetic amorphous alloy, or the like may be used, but these magnetic substances have a large switching field, so they cannot be easily applied to conventional MRAMs. In other words, for a large switching field, the width and thickness of an electrode line need to be greater than predetermined measures, so it is difficult to apply the above magnetic substances to memory devices requiring a high density of integration.
Meanwhile, when information is recorded in or reproduced from an array of memory devices using the sum of vectors in X- and Y-directions, two independent writing sources are needed, thereby complicating chip architecture.
In an effort to solve the problems described above, it is a feature of an embodiment of the present invention to provide a micro-magnetic random access memory (MRAM) device and array that do not require a switching field, are able to minimize power consumption, have a simple structure, can increase operating speed, and have a size in accordance with a magnetic domain size, thereby realizing a micro-memory device.
To provide the feature of the present invention described above, there is provided a magnetic random access memory including a data storage unit including a fixed layer, a non-magnetic layer, and a free layer having two ends; a data input means electrically connected to both ends of the free layer, for applying current to the free layer to input data into the data storage unit; and a data output means electrically connected to the free layer and the fixed layer to output data stored in the data storage unit.
Preferably, the data input means is a data input unit and the data output means is a data output unit.
Preferably, the fixed layer and the free layer are formed of a ferromagnetic substance, and the free layer includes at least one magnetic domain wall.
Preferably, the data input means includes a data input line through which current is applied to the free layer to determine a magnetization direction; and a data input selection switch formed between one end of the free layer and the data input line. Preferably, the data output means includes a bit line formed on the free layer, and a data output selection switch connected to a bottom of the fixed layer.
Preferably, each of the data input selection switch and the data output selection switch is a diode, a MOS transistor, or a bipolar transistor.
Another embodiment of the present invention also provides a magnetic random access memory array having a matrix structure of unit memory devices each including a data storage unit including a fixed layer, a non-magnetic layer, and a free layer having two ends; a data input means electrically connected to both ends of the free layer to input data into the data storage unit; and a data output means electrically connected to the free layer and the fixed layer to output data stored in the data storage unit. The data input means and the data output means are electrically connected to an input selection decoder and an output selection decoder, respectively, within the magnetic random access memory array to select a particular unit memory device.
Preferably, the data input means includes a data input line through which current is applied to the free layer to determine a magnetization direction, and a data input selection switch formed between one end of the free layer and the data input line and electrically driven by the input selection decoder. The data output means preferably includes a bit line formed on the free layer, and a data output selection switch connected to a bottom of the fixed layer and electrically driven by the output selection decoder.
Preferably, the magnetic random access memory array further includes a reference column composed of reference memory units in each of which the magnetization direction of a free layer of a data storage unit is fixed, and wherein bit lines of the unit memory devices and a bit line of the reference memory units are connected to a comparator.
In order to provide another feature of an embodiment of the present invention, there is provided a method of operating a magnetic random access memory array including an array having a matrix structure of unit memory devices each including a data storage unit having a fixed layer, a non-magnetic layer, and a free layer having two ends; a data input means electrically connected to both ends of the free layer to input data into the data storage unit; and a data output means electrically connected to the free layer and the fixed layer to output data stored in the data storage unit; and an input selection decoder and an output selection decoder electrically connected to the data input means and the data output means, respectively, to select a particular unit memory device in the array. The method includes (a) selecting a particular unit memory device using the input selection decoder, and forming a spin arrangement in a predetermined direction within a magnetic domain of the free layer of the data storage unit by applying current to the selected unit memory device using the data input unit, thereby storing data; and (b) driving an output selection switch connected to a particular unit memory device using the output selection decoder and reading data from the data storage unit using the data output unit.
Preferably, the data input means includes a data input line through which current is applied to the free layer to determine a magnetization direction, and a data input selection switch formed between one end of the free layer and the data input line and electrically driven by the input selection decoder. Here, (a) includes driving the data input selection switch using the input selection decoder, and forming a spin arrangement in a predetermined direction within a magnetic domain of the free layer through the data input line.
The data output means includes a bit line formed on the free layer, and a data output selection switch connected to a bottom of the fixed layer and electrically driven by the output selection decoder. Here, (b) includes driving the data output selection switch using the output selection decoder, and measuring a resistance value between the fixed layer and the free layer of the data storage unit to read data.
Preferably, the array further includes a reference column composed of reference memory units in each of which the magnetization direction of a free layer of a data storage unit is fixed, and bit lines of the unit memory devices and a bit line of the reference memory units are connected to a comparator. Here, (b) further includes comparing a resistance value of the data storage unit of the unit memory device with a fixed resistance value of the data storage unit of a reference memory unit corresponding to the unit memory device to output data stored in the unit memory device.